The Prime Minister says one of the many reasons he allegedly carried out Friday's shooting, which tragically left 50 people dead, was to raise his profile - and she doesn't want to let him have that small victory.

The alleged shooter's attempts to attract publicity are likely come during his appearances in court, and Jacinda Ardern says she will do what she can to stop him getting the attention he craves.

"We've agreed in principle on what we're going to do; we've got a bit of detail to work [through]," the Justice Minister said.

"I expect the Prime Minister, either today or tomorrow, will announce the actual detail of what the law change will be."

Mr Little appeared uncomfortable when asked if the attack could have been carried out if the gun laws were different, but admitted "it certainly would have been much harder".

He says while some of the new legislation will be implemented straight away, much of it will require "a proper law change process" that could take some time.

"There will be proper, open, normal Select Committee process, public submissions - that sort of thing."

The Justice Minister explained that Friday's atrocities show that with some "minor changes", it's simple to turn a Category A firearm into a high-powered, semi-automatic weapon.

"It's a gap in our law... The law change will mean that gap won't continue."

Meanwhile Mr Little is looking into whether the GCSB and SIS - the spy agencies he is the minister for - should have caught the alleged gunman before the attack was carried out.

"They feel, we all feel, a huge burden. The purpose of the inquiry is to ascertain whether they didn't see something they should've seen," he explained.

"We know they've got the technology; they've got the people; they've had their budget beefed up over the last three years. The question is - and it's not just the spy agencies, but Police, Customs, Immigration - whether they were blind to this threat.

"On the other hand, we know that the way this person operated, he wasn't hugely visible - this was an incredibly well-planned atrocity that he committed."

When asked whether it was embarrassing that it had taken so long for gun laws in New Zealand to change, Mr Little said that was "an obvious conclusion to draw".

"The events of Friday, I think we are still absorbing the magnitude of those events. I haven't fully absorbed the impact of it.

"But it does mean New Zealand is a different place, and if we want to hold onto the values that are important, and the freedoms we all enjoy, things will have to change."